Process for the preparation of sebacic acid

ABSTRACT

A process for preparing sebacic acid of high purity in a high yield from adipic acid by conducting the electrolytic condensation of monomethyl adipate at a high current efficiency and a low electric cell voltage. This process is characterized in that electrolytic condensation of monomethyl adipate and an alkali salt thereof is conducted by maintaining the dimethyl sebacate concentration in the electrolytic solution at 5% by weight or higher, and passing the electrolytic solution through an electrolytic cell at a rate of at least 2.5 m/sec, the extraction of dimethyl sebacate formed by the electrolysis is conducted with use of a combination of water and an organic solvent capable of dissolving dimethyl sebacate and insoluble in water, and the hydrolysis of the so recovered dimethyl sebacate is conducted by adding dimethyl sebacate to an aqueous solution of a mineral acid in a specific amount and conducting the hydrolysis while removing methanol.

United States Patent n91 Isoya et al.

[ PROCESS FOR THE PREPARATION OF SEBACIC ACID [75] Inventors: Toshiro lsoya; Rinichi Kakuta;

Chikayuki Kawamura, all of Nobeoka. Japan [73] Assignee: Asahi Kasei Kogyo Kabushiki Kaisha, Osaka, Japan [22] Filed: Jan. 24, 1974 [2]] Appl. No: 436,275

[30] Foreign Application Priority Data Jan. 31, 1973 Japan 48-11883 July 22, 1975 154,53l l/l962 U.S.S.R 204/59 Primary Examiner-F. C. Edmundson Attorney. Agent, or Firm-Burgess, Dinklage 8: Sprung [57] ABSTRACT A process for preparing sebacic acid of high purity in a high yield from adipic acid by conducting the elec trolytic condensation of monomethyl adipate at a high current efficiency and a low electric cell voltage. This process is characterized in that electrolytic condensa- Mar. 26, 1973 Japan c i r r. 48-34863 Och 3' [973 Japan N 48411218 tron of monomethyl adipate and an alkali salt thereof 17 1973 Japan H 48 x4]604 is conducted by maintaining the dimethyl sebacate concentration in the electrolytic solution at 5% by [52] Cl 204/59 R; 204/727 204/79 weight or higher, and passing the electrolytic solution [5 Int. Cl. C07: 5 through all l8ClfOlYtlC cell at a rate Of at least [58] Field of Search H 204; R 72 79 m/sec, the extraction of dimethyl sebacate formed by the electrolysis is conducted with use of a combination [56] Reerences Cited of water and an organic solvent capable of dissolving dimethyl sebacate and insoluble in water, and the hy- UNITED STATES PATENTS drolysis of the so recovered dimethyl sebacate is con- 2,439,425 4/1948 Gresham 204/72 ducted by adding dimethyl Sebacate to an aqueous S0 gi i "5 :5 :42 lution of a mineral acid in a specific amount and con- 3'787'299 M974 2 1 5; 204/59 R ducting the hydrolysis while removing methanol.

FOREIGN PATENTS OR APPLICATIONS 13 Claims, 3 Drawing Figures l,l8l,688 ll/l964 Germany 4. 204/59 20 /secl O i r i l P /crn cbsl PROCESS FOR THE PREPARATION OF SEBACIC ACID This invention relates to a process for preparing sebacic acid from adipic acid, and more particularly to a process for electrolytically preparing sebacic acid from monomethyl adipate and an alkali salt thereof.

The reaction of electrochemically condensing carboxylic acids is generally called the Kolbe reaction,

and this reaction is disclosed in such literature references as H. Kolbe, Ann., 69, 257 (i849) and A. C. Brown, Ann.,26l, I07 (I891). As a typical example of this reaction, there can be mentioned an electrolytic process for the preparation of dimethyl sebacate from monomethyl adipate. Monomethyl adipate is prepared by reacting adipic acid with methanol. In practicing this electrolytic reaction, monomethyl adipate is dissolved in methanol and a part of the adipate is neutralized by an alkali such as sodium hydroxide. In the methanol solution thus formed, there are co-existent monomethyl adipate and its alkali salt. Assuming that the molar amounts of the monomethyl adipate and the alkali salt thereof are A (moles) and B (moles), respectively,

A B (mole '36) is defined as degree a of neutralization" of the abovementioned methanol solution. When the so formed According to the conventional techniques, it has been considered that in order to obtain a high electric current efficiency it is necessary to maintain the degree a of neutralization of monomethyl adipate at a low level. For example, Japanese Pat. Application No. 37564/71 teaches that it is indispensable to maintain the degree a of neutralization below 20 mole However, if such low degree of neutralization is adopted, there is a disadvantage that the electrolysis voltage becomes high.

Accordingly, if a high current efficiency can be obtained under a low electrolysis voltage in the industrial practice of this electrolytic reaction, there will be attained great advantages.

As a result of our investigations we have now completed an advantageous process in which a very high electric current efficiency can be attained while the voltage of an electrolytic cell is maintained at a low level.

More specifically, it has been found that when the composition of the electrolytic solution is so controlled that the concentration of dimethyl sebacate formed by the electrolytic reaction is at least by weight, a high electric current efficiency can be obtained, and even if the degree a of neutralization exceeds mole the reaction can be performed with a high electric current efficiency and a low electrolytic cell voltage.

Provision of an electrolytic call of an appropriate size is required for practicing the process on an industrial scale by employing the above-mentioned electrolytic solution while the inside pressure of the cell is determined by the volume and structure thereof. For example, in the case of an electrolytic cell comprising parallelly disposed cathode and anode plates between which an electrolytic solution is passed, which is usually employed in organic electrolytic reactions, if the process is conducted on an industrial scale, it is desired that the flow passage of the electrolytic solution is within a range of from 50 to 200 em, but if the flow passage of the electrolytic solution is longer, the inside pressure of the cell becomes higher and there is caused such a dis advantage that the current efficiency is lowered.

As a result of our further investigations made with a view to overcoming this defect, it has surprisingly been found that if the electrolytic solution is passed between the cathode and anode plates at a flow rate of at least 2.5 m/sec, the electrolytic condensation of monomethyl adipate can be accomplished with a high electric current efficiency in spite of increase of the inside pressure of the electrolytic cell.

As known means for separating from the resulting electrolytic solution dimethyl sebacate formed by the above electrolytic reaction, there can be mentioned distillation, crystallization and extraction. However, the distillation method is defective in that azeotropy is caused between dimethyl sebacate and monomethyl adipate and they cannot be separated from each other. Further, in the crystallization method, there are such disadvantages that the operation should be conducted at a low temperature and occlusion of monomethyl adipate in the resulting crystals cannot be avoided. in the case of the extraction method, if water is employed as an extraction solvent, it is necessary to employ it in a very large amount, and in general, water should be used in an amount of at least 3 parts per part of the residual liquid left after removal of methanol from the electrolytic solution. Moreover, since the difference of the specific gravity between the extracted water layer and the oil layer is extremely small, it is very difficult to separate the two layers from each other. In case an organic solvent such as n-heptane is employed as an extraction solvent, entrainment of a certain amount of monomethyl adipate in the n-heptane layer cannot be avoided and therefore, it is impossible to separate only dimethyl sebacate as the product. As is seen from the foregoing explanation, the most important problem at the step of separating dimethyl sebacate from the electrolytic solution is how to separate it from the starting monomethyl adipate.

As a result of our further investigations made with a view to overcoming these disadvantages involved in separation to dimethyl sebacate from the electrolytic solution, it has been found that when water and an organic solvent capable of dissolving dimethyl sebacate but insoluble in water are simultaneously added to a dimethyl sebacate-containing electrolytic solution formed by the electrolytic condensation of monomethyl adipate in methanol or when such organic solvent is added to the electrolytic solution to form two layers and water is added to the organic solvent layer to make water copresent with the organic solvent, the system is separated into water and organic solvent layers and dimethyl sebacate is extracted into the organic solvent layer, whereby dimethyl sebacate free of monomethyl adipate can be recovered. Impurities contained in the dimethyl sebacate so recovered can be removed, according to necessity, by distillation or the like.

Hydrolysis of dimethyl sebacate with a mineral acid for formation of sebacic acid is accomplished according to the conventional technique by adding water in an amount of at least 4 parts by weight per part by weight of the dimethyl sebacate layer prior to the acid hydrolysis, in order to prevent the formed methanol and sebacic acid from undergoing reaction under the action of the hydrolysis agent, and carrying out the hydrolysis reaction at a temperature higher than 100C. under an elevated pressure. However, such hydrolysis process involves complicated steps. resulting in great disadvantages when the process is carried out on an industrial scale.

As a result of our further investigations made with a view to developing a hydrolysis process overcoming these disadvantages, it has been found that when dimethyl sebacate is added to an aqueous solution of a mineral acid in the presence of dimethyl sebacate in an amount smaller than the amount corresponding to the saturation solubility of sebacic acid in said mineral acid solution and the hydrolysis is carried out while removing methanol formed by the reaction, the hydrolysis can be completed quite easily.

As is apparent from the foregoing explanation, according to this invention, preparation of sebacic acid from adipic acid can be performed on an industrial scale with great advantages.

Features of this invention will be apparent from the following description with reference to the accompanying drawing, in which:

FIG. 1 is a curve illustrating the relation between the inside pressure of the electrolytic cell and the electric current efficiency;

FIG. 2 is a curve illustrating the relation of the liquid velocity in the electrolytic cell to the electric current efficiency, the inside pressure of the electrolytic cell and the electrolytic cell voltage; and

FIG. 3 is a flow sheet showing an embodiment of this invention, in which reference numerals l, 2 and 4 indicate an electrolytic solution tank, an electrolytic cell and an extraction column. respectively.

Monomethyl adipate, which is used as the starting material in the process of this invention, is prepared from adipic acid and methanol. in this esterification reaction, it is preferred that water is present in the reaction mixture in an amount of at least 1 mole per mole of adipic acid present in the reaction mixture.

In order to know the relation of the concentration of dimethyl sebacate to the electric current efficiency and the cell voltage, the following experiments were conducted.

Various electrolytic solutions were prepared by neutralizing a methanol solution of monomethyl adipate and dimethyl sebacate with sodium hydroxide so that the concentration of monomethyl adipate was 20% by weight and the neutralization degree a was 30 mole and by changing the concentration of dimethyl sebacate as shown in Table 1 given below. Either a cathode plate or an anode plate had a current-applied area of 2 X 50 cm. The cathode was composed of a SUS 27 plate having a thickness of 2 mm and the anode was composed of a titanium plate of a thickness of 2 mm plated with platinum in a thickness of 2 microns. A polyethylene plate of a thickness of 2 mm s perforated that the current-applied area was 2 X 50 cm was disposed between both the electrodes and the electrode distance was adjusted to 2 mm. An electrolytic cell used had inlet and outlet openings for the electrolytic solution, and the electrolytic solution was passed through both the electrodes at a liquid velocity of 2 m/sec at a current density of 20 A/dm. The temperature of the electrolytic solution at the cell outlet was maintained at 55C. The electrolysis'was conducted for l0 hours under theiforegoing conditions while removing a part ofthe'electrolytic solution and supplying methanol. monomethyl adipate and sodium monomethyl adipateso that the concentration of dimethyl sebacate was always maintained at the-prescribed level. Results are showhin Table l. from which it is seen that a high electric current efficiency can be obtained when the concentrationjof dimethyl sebacate in the electrolytic solution is at last 5% by weightliflow'ever, in view of the fact that wl'ien the concentration of dimethyl sebacate is. too high, the cell voltage is increased, it is preferred that the concentration of dimethyl sebacate in the electrolytic solution is within a range of from 5 to 40% by weight.

" t I Table 1 Concentration of dimethyl sebacate in electrolytic Yield of Electric Cell solution dimethyl current voltage by weight) sebacate (96) efficiency (In (volt) The above eiiperiments were repeated in the same manner except that the concentration of dimethyl sebacate was maintained at 20% by weight and the neutralization degree a was varied as indicated in Table 2. Results are shown in'Ta-ble 2, from which it is seen that when the neutralization degree a is up to about 60 mole the electric current efficiency can be maintained at an almost constant high level and the neutralization degree exceeds 60'mqle the electric current efficiency is gradually lowered. It is also seen that when the degree a of neutralization of monomethyl adipate is higher than 20 mole a cell voltage is maintained at an almost constantlow level.

Table 2 Degree A of neutralization of Yield of Electric Cell monethyl adipate dimethyl current voltage (mole sebacate efficiency (35) (volt) 78.4 sat l3.6

From theresults shown in Tables l and 2, it is seen that when the concentration of dimethyl sebacate in the electrolytic solution is at least 5% by weight, preferably 5 to 40% by weightiand the degree a of neutralization of monomethyl adipateis at least 20 mole preferably 20 to 60 mole ahigh'electric current efi'iciency can be obtained'at the electrolysis and it is made possible to conduct the electrolysis at a low cell voltage.

In order to increase the electric conductivity of the electrolytic solutionja neutralizing base is employed. Examples of the neutralizing base include hydroxides, carbonates, methylates and ethylates of lithium, sodium and potassium, ammonia, dimethylamine, trimethylamine, ethylamine, ethanolamine and the like.

Methanol is most preferred as a reaction solvent, but ethanol, isopropanol, ethylene glycol, dimethylformamide, diethylformamide, dimethylacetamide, diethylacetamide, acetonitrile, propionitrile, tetrahydrofuran and water can be used singly or as one component of a mixed solvent.

Provision of an electrolytic cell of a certain size is required for conducting the electrolysis on an industrial scale with use of the above-mentioned electrolytic solution, and the inside pressure of the electrolytic cell used is determined by the volume and structure of the cell. In an electrolytic cell customarily used for the organic electrolysis reaction, cathode and anode plates are disposed in parallel to each other and an electrolytic solution is passed between the two electrodes. When the electrolysis is conducted on an industrial scale by employing such conventional cell, it is desired that the flow passage of the electrolytic solution is within a range of from 50 to 200 cm. However, if the flow passage of the electrolytic solution is too long, the inside pressure of the electric cell is increased and hence, there is caused an undesired reduction of the electric current efficiency. The relationship between the inside pressure of the electric cell and the electric current efficiency is known in the art. For reference, results of experiments made with a view to demonstrating this relationship are described below.

An electrolytic solution containing 20% by weight of monomethyl adipate and 20% by weight of dimethyl sebacate, the balance being methanol, in which the degree a of neutralization of monomethyl adipate with potassium hydroxide was 30 mole was electrolyzed at an electrolytic solution temperature of 55C. at an electrolytic solution flow rate of l m/sec for 2 hours by employing an electrolytic cell having an electrolytic solution flow passage of alength of I00 cm and a width of IO cm and an electrode distance of 2 mm. The obtained results are shown in Table 3.

From the foregoing results, it is seen that increase of the inside pressure of the electrolytic cell results in large reduction of the electric current efficiency.

ln order to prevent increase of the inside pressure of the electrolytic cell, it has been tried to reduce the flow rate of the electrolytic solution or broaden the distance between the electrodes. However, these trials have ended in failure. More specifically, if the flow rate of the electrolytic solution is reduced the inside pressure of the electrolytic cell is lowered, but the electric current efficiency is simultaneously lowered. When the distance between the electrodes is broadened, there is another disadvantage that the electric pressure in the electric cell is increased. With a view to overcoming these disadvantages, we have made experiments to examine the relationship between the inside pressure (p) of the electrolytic cell and the electric current efficiency (1') with respect to various liquid velocities (LV) of the electrolytic solution. Results are shown in FIG. 1. As is apparent from the results, when the liquid velocity, i.e., the flow tate, of the electrolytic solution is increased, even if the inside pressure of the cell is high, the electric current efficiency can be maintained at a high level and it is hardly influenced by the inside pressure of the cell. Results of experiments in which the relationship between the liquid velocity (LV) of the electrolytic solution, the inside pressure (p) of the electrolytic bell, the electric current efficiency (1) and the voltage (V) of the electrolytic cell was examined while maintaining the outlet pressure of the cell at 1 atm., are shown in FIG. 2. From these results, it will readily be understood that although with increase of the liquid velocity of the electrolytic solution the inside pressure of the electrolytic cell increases, if the liquid velocity of the electrolytic solution is maintained at a level of at least 2.5 m/sec, a high electric current efficiency can be attained and the voltage of the electrolytic cell can be maintained at a very low level. in short, if the flow rate of the electrolytic solution in the electrolytic cell is adjusted to at least 2.5 m/sec, preferably 2.5 to 4 m/sec, the electrolysis can be accomplished with an unexpectedly high current efficiency and at a very low voltage of the electrolytic cell.

Accordingly, in this invention, any of the electrolytic cells customarily used for organic electrolytic reactions can be used as far as it has such a structure that the electrolytic solution can pass between the electrodes at such a high flow rate as mentioned above. For instance, it is possible to employ an electrolytic cell comprising parallelly disposed cathode and anode plates and a polyethylene plate disposed between the electrodes to define the electrode distance, in which a hole is perforated at the central portion of the polyethylene plate so that the electrolytic solution can pass therethrough. In this cell, the current-applied area is determined by the size of this hole and the electrode distance is defined by the thickness of the polyethylene plate. The electrolytic solution is fed from an inlet opening, and a part of the electrolytic solution is reacted while it passes through the space between the electrodes. Then, the electrolytic solution is withdrawn from an outlet opening and recycled to an electrolytic solution tank.

As a material for the anode, there are employed, for example, platinum, ruthenium, rhodium, palladium, gold and alloys thereof. In general, these metals are used in the form as plated on a substrate such as titanium, tantalum and iron.

As a material for the cathode, a substance having a low hydrogen overvoltage is preferably used but the material for the cathode is not particularly critical in this invention. Platinum, iron, nickel, stainless steel and the like may preferably be employed.

It is preferred that the distance between the cathode and the anode is within a range of from I to 3 mm. in general, it is appropriate that the length of the flow passage of the electrolytic solution is within a range of from 50 to 200 cm. It is desired that the temperature of the electrolytic solution is higher than 40C., but it should be lower than the boiling point of the electrolytic solution. The current density is not particularly critical in this invention but it is generally preferred that the electrolysis is carried out at a current density of 20 to 40 A/dm Dimethyl sebacate thus prepared should be separated from the electrolytic solution. This separation will now be illustrated by reference to the flow sheet of FIG. 3. In FIG. 3, numeral 1 represents an electrolytic solution tank. and the electrolytic solution is circulated to an electrolytic cell 2. The electrolytic solution is a methanol solution containing monomethyl adipate, dimethyl sebacate and minor amounts of such by-products as dimethyl adipate, methyl n-valerate, methyl w-hydroxyvalerate and methyl allylacetate. A part of the electrolytic solution is withdrawn and methanol is removed at a distillation column 3 while the remainder is forwarded to an extraction column 4. In this invention, the withdrawn electrolytic solution may be forwarded to the extraction column 4 without removal of methanol. An organic solvent is fed from a feed inlet 5 and for warded to the lower portion of the extraction column 4 together with the electrolytic solution. The feed inlet 5 for an organic solvent may be provided at a position other than the point shown in FIG. 3, for example, at a lower or middle portion of the extraction column. Any organic solvent capable of dissolving dimethyl sebacate and insoluble in water can be used but use of nhexane, n-heptane, n-octane, cyclohexane, iso-octane, etc. is preferred. The organic solvent layer containing dimethyl sebacate overflowing from the head of the extraction column 4 is fed to a distillation column 7 where the organic solvent fed from the feed inlet 5 is distilled and is used again for extraction. A liquor composed mainly of dimethyl sebacate is recovered from the bottom portion of the column 7, and according to need, it is forwarded to another distillation column (not shown) to obtain a higher purity product of dimethyl sebacate. An aqueous solution containing monomethyl adipate and its alkali salt is withdrawn from the bottom portion of the extraction column 4 and is fed to a distillation column 8, where water is distilled from the column head and is used again for extraction. Monomethyl adipate and its alkali salt are withdrawn from the bottom portion of the column 8, and are circulated to the electrolytic solution tank and used for the electrolysis reaction. Water and organic solvent are used for the extraction in such amounts as sufficient to form an organic layer and an aqueous layer. In general, water is used in an amount of 0.2 to 2 parts by weight per part by weight of the electrolytic solution to be extracted and the organic solvent is used in an amount of 0.2 to 2 parts by weight per part by weight of the electrolytic solution. In this invention, it is also possible to perform the extraction by adding an organic solvent to the electrolytic solution to be extracted to form two layers, withdrawing the organic solvent layer, adding water thereto and effecting the extraction in the copresence of water and the organic solvent. Results of our experiments made on extraction of dimethyl sebacate from the electrolytic solution are now described.

The electrolytic solution to be extracted was a methanol solution containing 30% by weight of dimethyl sebacate, 20% by weight of monomethyl adipate, 9% by weight of sodium monomethyl adipate and minor amounts of such by-products as dimethyl adipate, methyl nwalerate, methyl w-hydroxyvalerate and methyl allylacetate, and methanol was removed from this electrolytic solution by distillation. Then, 500 g. of water was added to lOO g. of the residual liquid and the mixture was agitated at room temperature to form two layers. The upper oil layer contained mainly dimethyl sebacate and monomethyl adipate and the lower aqueous layer contained mainly monomethyl adipate and its sodium salt. The ratios (C /C of the concentrations (C of monomethyl adipate and dimethyl adipate in their upper layer to the concentrations (C in the lower layer were determined. The results are shown in Table 4. The above procedures were repeated in the same manner by employing 500 g. of n-heptane instead of water. In this case, the upper n-heptane layer contained dimethyl sebacate and monomethyl adipate and the lower layer contained mainly monomethyl adipate and its sodium salt. The results are shown in Table 4.

As is seen from the foregoing results, in the extraction with an organic solvent, even if counter-current extraction is carried out with an increased number of stages, a certain amount of monomethyl adipate is inevitable contained in the organic solvent layer and it is impossible to obtain dimethyl sebacate of a high purity. According to this invention, the foregoing defect involved in the extraction step can be effectively overcome. More specifically, as is seen from the results obtained by adding water and n-heptane to g of the residual liquid left after removal of methanol, and agitating the mixture at room temperature to separate it into two layers, which are also shown in Table 4, the upper n-heptane layer comprises dimethyl sebacate alone and the lower aqueous layer comprises predominantly monomethyl adipate and its sodium salt. It will readily be understood that if water and an organic solvent capable of dissolving dimethyl sebacate and insoluble in water are used in combination for extraction, dimethyl sebacate of very high purity can be obtained with use of a small amount of the extraction solvent by, for example, the counter-current extraction.

in Table 4, the purity is determined from the concentration (Cs) of dimethyl sebacate and the concentration (Ca) of monomethyl adipate in the upper layer according to the following formula:

Purity Cs/(Cs Ca) X I00 As is apparent from the results shown in Table 4, if water and an organic solvent capable of dissolving dimethyl sebacate and insoluble in water are both present in the extraction step, dimethyl sebacate can be separated from the electrolytic solution without entrainment of monomethyl adipate, and by-products contained in the separated dimethyl sebacate, such as dimethyl adipate, methyl n-valerate, methyl w-hydroxyvalerate and methyl allylacetate can easily be separated from dimethyl sebacate by, for example, distillation. Thus, according to this invention, it is possible to recover dimethyl sebacate of such high purity as it not attainable by use of water or an organic solvent alone.

The dimethyl sebacate so separated is hydrolyzed with use of a mineral acid to obtain sebacic acid. Also in connection with this hydrolysis, we have found that the operation can be performed very advantageously if the reaction conditions are appropriately controlled. More specifically, it has been found that in the hydrolysis of dimethyl sebacate with a mineral acid, if the hydrolysis is carried out while removing methanol formed by the reaction from the reaction system, the hydrolysis can be greatly accelerated and advanced very promptly. However, when the hydrolysis is carried out merely by removing methanol from the reaction system, unreacted dimethyl sebacate and monomethyl sebacate are incorporated into precipitated crystals of sebacic acid, and it takes a long time to complete the hydrolysis reaction. As a result of our investigations, it has been found that if dimethyl sebacate is added to an aqueous solution of a mineral acid in an amount not exceeding the amount corresponding to the saturation solubility of sebacic acid in the mineral acid aqueous solution at the hydrolysis temperature and the hydrolysis is carried out while removing methanol, the hydrolysis reaction can be completed in a very short time. In the conventional process, it is necessary to conduct the hydrolysis under high temperature and high pressure conditions prior to the hydrolysis with an aqueous solu tion of a mineral acid. In contrast, such preliminary hydrol"sis need not be effected in this invention, and the hydrolysis of dimethyl sebacate to sebacic acid can easily be accomplished in a short time by adding dimethyl sebacate to an aqueous solution of a mineral acid in a specific amount and conducting the hydrolysis while removing methanol.

When dimethyl sebacate is added to an aqueous solution of a mineral acid in an amount not exceeding the amount corresponding to the saturation solubility of sebacic acid in the mineral acid aqueous solution, all the sebacic acid formed by the hydrolysis is dissolved in an aqueous mineral acid solution and it is not precipitated in the form of crystals. Accordingly, the hydrolysis reaction can be accomplished promptly without incorporation of unreacted dimethyl sebacate or monomethyl sebacate into sebacic acid. When the hydrolysis is carried out by removing methanol formed by the reaction from the reaction system, the hydrolysis reaction can be accelerated, and modification of methanol with the mineral acid can be effectively prevented. The removal of methanol may be effected by, for example, distillation. It is preferred that the methanol concentration in the liquor to be hydrolyzed is low, and in general it is maintained below 0. l% by weight.

Nitric acid is preferred as the mineral acid, but sulfuric acid, hydrochloric acid and the like can also be employed. It is preferred that the acid concentration is within a range of from l to 30% by weight. The hydrolysis is conducted at a temperature of at least 90C., preferably at least 100C. Generally, the hydrolysis is conducted at a temperature of 100 120C.

Effects attained by this invention are as follows:

I. In the electrolytic condensation of monomethyl adipate and its alkali salt in a methanol solution, by making dimethyl sebacate present in the electrolyte in an amount of at least by weight, it is made possible to prepare dimethyl sebacate with a high current efficiency.

2. In the electrolytic condensation of monomethyl adipate and its alkali salt in a methanol solution, by passing the electrolytic solution in an electrolytic cell at a liquid velocity of at least 2.5 m/sec, it is made possible to prepare dimethyl sebacate at a low electrolytic cell voltage with a high electric current efficiency.

3. When dimethyl sebacate is formed by the electrolytic condensation of monomethyl adipate and its sodium salt in a methanol solution and when water and an organic solvent capable of dissolving dimethyl sebacate and insoluble in water are added to the resulting electrolytic solution or such organic solvent alone is added to the resulting electrolytic solution to form two layers and water is added to the organic solvent layer, thereby to separate the electrolytic solution into aqueous and organic solvent layers and extract dimethyl sebacate into the organic solvent layer, it is possible to separate and recover dimethyl sebacate of very high purity.

4. in preparation of sebacic acid by hydrolyzing dimethyl sebacate formed by the electrolytic condensation of monomethyl adipate and its alkali salt in a methanol solution, if dimethyl sebacate is added to an aqueous solution ofa mineral acid to be used for the hydrolysis in an amount not exceeding the amount corresponding to the saturation solubility of sebacic acid in the mineral acid aqueous solution at the hydrolysis temperature and the hydrolysis is carried out while removing methanol formed by the reaction from the reaction system, the hydrolysis reaction can be completed in a short time with ease and without unfavorable modification of the methanol and the desired product, i.e. sebacic acid.

As is seen from foregoing explanation, according to this invention it is possible to prepare sebacic acid from adipic acid with great advantages.

This invention will now be illustrated in more detail by reference to the following Examples.

EXAMPLE l 10 Kg of a methanol solution containing 20% by weight of monomethyl adipate, which was neutralized with sodium hydroxide so that the neutralization degree a was 30 mole and 20% by weight of dimethyl sebacate was stored as an electrolytic solution in an electrolytic solution tank and was circulated into an electrolytic cell.

Both the electrodes of the cell had a current-applied area of2 X 50 cm. The cathode was composed of a SUS 27 plate having a thickness of 2 mm and the anode was composed of titanium plate of a thickness of 2 mm plated with platinum in a thickness of 2 u. A polyethylene plate of a thickness of 2 mm perforated so that the current-applied area was 2 X 50 cm was disposed between both the electrodes to define the electrode distance as 2 mm. The electrolytic cell had inlet and outlet openings for the electrolytic solution and the electrolytic solution was passed between both the electrodes at a liquid velocity of 2 m/sec. The electrolysis was carried out at a current density of 20 A/dm and an electrolytic solution temperature of C. for 10 hours. The concentration of dimethyl sebacate in the electrolytic solution withdrawn after the electrolysis reaction was determined by gas chromatography and the yield was calculated to obtain results shown in Table 5, The above procedures were repeated in the same manner by changing the kind of the base used for neutralization to obtain results shown in Table 5.

Table Yield of Electric Cell dimethyl current voltage Base sebacate efficiency (91:) (volt) LiOl-l 93.8 92.6 14.2 NIOH 91.7 90.9 14.5 KOH 91.3 90.5 14.4 NaOCl-l, 89.8 88.5 16.0 NH, 91.: 90.8 13.3

EXAMPLE 2 The Kolbe synthesis reaction was carried out under the same conditions as adopted in Example 1 by using sodium hydroxide as the neutralization base, adjusting the neutralization degree a to 30 mole and changing the current density as indicated in Table 6, Results are shown in Table 6.

Table 6 Current Yield of Current Cell density dimethyl Efficiency voltage (A/dm sebacate (70) (volt) EXAMPLE 3 The Kolbe synthesis reaction was carried out under the same conditions as adopted in Example 1 by employing sodium hydroxide as the neutralization base and changing the concentration of monomethyl adipate as indicated in Table 7. Results are shown in Table 7.

5 Kg of a solution containing 30% by weight of monomethyl adipate, which was neutralized with sodium hydroxide so that the neutralization degree a was 40 mole and by weight of dimethyl sebacate, the balance being methanol, was stored as an electrolytic solution in an electrolytic solution tank and was circulated in an electrolytic cell.

A cathode was composed of a titanium plate of a thickness of 2 mm and an anode was composed of a titanium plate of a thickness of 2 mm plated with platinum in a thickness of 3 t. Both the electrodes had a current-applied area of 1 X 150 cm, and a polyester plate perforated so that it had the above current applied area was disposed to define the electrode distance as 3 mm. The electrolytic cell had inlet and outlet LII openings for -the.e1ectrolytic solution and the electro lytic solution was flown between the electrodes at a liquid velocity of 3 rnm/sec. The electrolysis was conducted at a current density of 25 A/dm and an electrolytic solution temperature of 60C. at the outlet of the cell. The average inside pressure of the electrolytic cell was 1.31 atm. and the electrolytic cell voltage was 1 volts.

After termination of the electrolysis reaction, the amount of formed dimethyl sebacate was determined by gas chromatography from which it was found that the electric current efficiency was 83%.

EXAMPLE 5 With use of 5 Kg of an electrolytic solution containing 40% by weight of monomethyl adipate, which was neutralized with sodium hydroxide so that the neutralization degree a was 10 mole and 20% by weight of dimethyl sebacate, the balance being methanol. the electrolysis was carried out for 2 hours under the same conditions as adopted in Example 4 except that the liquid velocity of the electrolytic solution. the length of the electrolytic solution flow passage and the electrode distance were changed to 4 m/sec. cm and 2 mm, respectively. The inside pressure of the electrolytic cell was 1.48 atm. and the electrolytic cell voltage was 17 volts.

The amount of dimethyl sebacate formed was determined by gas chromatography, from which it was seen that the electric current efficiency was 88%.

EXAMPLE 6 With use of 5 Kg of an electrolytic solution containing 10% by weightof monomethyl adipate, which was neutralized with sodium hydroxide so that the neutralization degree a was 50 mole and 30% by weight of dimethyl sebacate, the balance being methanol, the electrolysis was carried out under the same conditions as adopted in Example 5 except that the liquid velocity of the electrolytic solution in the cell was changed to 2.5 m/sec. The inside pressure of the electrolytic cell was 1.25 atm. and the electrolytic cell voltage was 14 volts.

The amount of dimethyl sebacate formed was determined by gas chromatography, from which it was found that the current efficiency was 81%.

EXAMPLE 7 An electrolytic solution containing 15.5% by weight of monomethyl adipate, 7.3% by weight of potassium monomethyl adipate, 22.1% by weight of dimethyl sebacate and 2.5% by weight of other by-products, the balance being methanol, was withdrawn from an electrolytic cell without removal of methanol from the electrolytic solution. Then, 100 g of water and 100 g of cyclohexane, n-hexane, n-heptane or isooctane were added to 200 g of the so withdrawn electrolytic solution, and the mixture was agitated at 40C. to separate it into two layers. Results are shown in Table 8 in the same man- Table S-Continued Organic Solvent C /C; cyclohexane n-hexane n-heptane isooctane dfitethyl 13.3 10.7 14.3 8.7 sebacate purity (52) 98.3 97.9 98.3 97.9

EXAMPLE 8 The extraction operation was conducted by employing. a counter-current extraction column having an inner diameter of 35 mm and a column length of 1200 mm. An electrolytic solution containing 12.7% by weight of monomethyl adipate, 5.2% by weight of potassium monomethyl adipate, 19.5% by weight of dimethyl sebacate and 2.4% of other by-products, the balance being methanol, extracted. A mixture of 1 part by weight of the above electrolytic solution and a part by weight of cyelohexane was fed under agitation at a rate of 5 m/hr from the bottom portion of the extraction column, and water was fed at a rate of 1 m/sec from the head portion of the extraction column, thereby to effect the counter-current extraction at room temperature.

The concentration of dimethyl sebacate in the cyclohexane layer flowing from the upper portion of the column was 22.8% but the concentration of monomethyl adipate was lower than 0.01% by weight. The concentration of dimethyl sebacate in the aqueous layer withdrawn from the lower portion of the column was 2.1% by weight but the concentration of monomethyl adipate was 8.9% by weight.

As is apparent from the above results, by employing water and cyclohexane as the extraction solvent, dimethyl sebacate alone could be separated without substantial incorporation of monomethyl adipate.

Water was removed from the aqueous layer by distillation, and the residual liquor was returned to the elec trolytic solution tank and subjected to the electrolysis.

EXAMPLE 9 Methanol was removed by distillation from an electrolytic solution containing 20.4% by weight of dimethyl sebacate, 10.0% by weight of monomethyl adipate, 3.5% by weight of potassium monomethyl adipate and 3.5% by weight of other by-products, the balance being methanol. Then, 500 g of n-heptane was added to 500 g of the residual liquor, and the mixture was agitated at room temperature to separate it into two layers. The upper layer contained 25.8% by weight of dimethyl sebacate and 1.6% by weight of monomethyl adipate. Then, 100 g of water was added to 100 g of this n-heptane layer and the mixture was agitated to separate it into two layers. The upper n-heptane layer contained 25.3% by weight of dimethyl sebacate and less than 0.01% by weight of monomethyl adipate, and the lower aqueous layer contained 1.9% by weight of monomethyl adipate and less than 0.01% by weight of dimethyl sebacate. From these results, it can readily be understood that dimethyl sebacate could be separated from monomethyl adipate quite effectively in this Example.

The lower layer obtained at the first extraction step was an oil layer containing 41.4% by weight of monomethyl adipate, 23.5% by weight of dimethyl sebacate, 19.6% by weight of n-heptane and potassium monoethyl adipate. After removal of n-heptane from the oil layer by distillation, the residual liquor was returned to the electrolytic solution tank and subjected to the electrolysis.

EXAMPLE 10 A 2 liter capacity glass vessel was charged with 100 g of dimethyl sebacate formed by the electrolysis and 1.5 Kg of an aqueous solution containing 18% by weight of nitric acid, and the resulting mixture was violently agitated and refluxed under atmospheric pressure. ln this manner, the hydrolysis was carried out for 3 hours while removing methanol formed by the hydrolysis reaction by distillation. At the time of termination of the reaction, the rate of hydrolysis was 99.9 mole The methanol concentration in the resulting liquor was maintained below 0.03% by weight and sebacic acid formed by the hydrolysis was dissolved in the reaction medium and was not precipitated.

EXAMPLE 1 1 With use of 100 g of dimethyl sebacate formed by the electrolysis and 1 Kg of an aqueous solution containing 24% by weight of nitric acid, the hydrolysis was carried out for 5 hours in the same manner as in Example 10. The obtained degree of hydrolysis was 97.2 mole the methanol concentration in the resulting liquor was maintained below 0.03% by weight and all the formed sebacic acid was dissolved in the reaction medium without any deposition.

EXAMPLE 12 With use of 100 g of dimethyl sebacate formed by the electrolysis and 1 Kg of an aqueous solution containing 18% by weight of nitric acid, the hydrolysis was carried out in the same manner as in Example 10. The obtained degree of hydrolysis was 94.8 mole The methanol concentration in the resulting liquid was 0.05% by weight and all the formed sebacic acid was dissolved in the reaction medium.

The so formed liquor was cooled to room temperature to precipitate crystals of sebacic acid, which were recovered by filtration, washed with 100 g of water and dissolved in 200 g of water at 130C. under a pressure of 2.7 Kg/cm to effect recrystallization. The resulting crystals were recovered by filtration, washed with 100 g of water and then dried to obtain g of pure sebacic acid having a melting point of 134C.

EXAMPLE 13 A mixture comprising 1 mole of adipic acid, 5 moles of water, 5 moles of methanol and 0.2 mole of dimethyl adipate was reacted under atmospheric pressure for 5 hours to form monomethyl adipate. The reaction mixture was subjected to distillation and dimethyl adipate was separated.

10 Kg of a methanol solution containing 20% by weight of monomethyl adipate, which was neutralized with potassium hydroxide so that the neutralization degree was 30 mole 20% by weight of dimethyl sebacate and 2% by weight of other by-products was charged into an electrolytic solution tank and it was subjected to the electrolysis.

Both the electrodes of an electrolytic cell used had a current-applied area of 2 X cm. The cathode was composed of a SUS plate having a thickness of 2 mm and the anode was composed of a titanium plate of a thickness of 2 mm plated with platinum in a thickness of2 pt. A polyethylene plate ofa thickness of 2 mm perforated so that it had a current-applied area of 2 X 100 cm was disposed between both the electrodes to define the electrode distance as 2 mm. The electrolytic cell had inlet and outlet openings for the electrolytic solution, and the electrolytic solution was passed between both the electrodes at a liquid velocity of 3 m/sec. The electrolysis was conducted at a current density of Aldm at an electrolytic solution temperature of 55C.

at the outlet of the cell. While withdrawing a part of the electrolytic solution and supplying fresh methanol, monomethyl adipate and its potassium salt so that the above-mentioned composition of electrolytic solution was always maintained, the electrolysis was continued for 49 hours. The amount of formed dimethyl sebacate was determined by gas chromatography, from which it was found that the electric current efficiency was 85.7% and the yield of the product was 89.2%. During the electrolysis the average cell voltage was 13.3 volts.

1n the withdrawn electrolytic solution, the dimethyl sebacate concentration was 21% by weight, the monomethyl adipate concentration was 18% by weight, and the neutralization degree was 27 mole In the same manner as described in Example 8, 10 Kg of this withdrawn electrolytic solution was subjected to countercurrent extraction to separate dimethyl sebacate therefrom. in the extracted cyclohexane layer, the dimethyl sebacate concentration was 23% by weight and the monomethyl adipate concentration was lower than 0.01% by weight.

The so recovered cyclohexane solution was subjected to fractional distillation to remove low-boiling-point components such as cyclohexane, methanol, water, methyl n-valerate, methyl w-hydroxyvalerate, methyl allylacetate and dimethyl adipate, and then, dimethyl sebacate was distilled and purified. The amount of the so recovered dimethyl sebacate was 1.4 Kg, and the product had a purity higher than 99.9% and a melting point of 264C.

With use of 0.5 Kg of dimethyl sebacate formed by the electrolysis in the foregoing manner and 1.5 Kg of 25% nitric acid, hydrolysis was conducted for 4 hours in the same manner as described in Example 10. At the time of termination of the reaction, the degree of hy drolysis was higher than 99.9 mole and the methanol concentration in the resulting liquor was 0.01% by weight. The so formed reaction liquor was subjected to crystallization in the same manner as in Example 12 to obtain 0.41 Kg of pure sebacic acid having a melting point of l34C.

What is claimed is:

1. In a process for preparing dimethyl sebacate comprising subjecting to an electrolytic condensation in an electrolytic cell a solution of monomethyl adipate and an alkali salt thereof in methanol, the improvement which comprises maintaining the concentration of dimethyl sebacate in the solution at a level of at least 5% by weight, and passing the solution through the cell at a liquid velocity of at least 2.5 m/sec.

2. The improved process according to claim 1 wherein the concentration of dimethyl sebacate is maintained at 5 to 40% by weight.

3. The improved process according to claim 1 wherein the degree of neutralization of said solution is at most 60 mole 4. The improved process according to claim 1 wherein the liquid velocity of the solution is 2.5 to 4.0 m/sec.

5. The improved process according to claim 1 wherein the concentration of dimethyl sebacate in the solution is 4 to 40% by weight.

6. In a process for preparing dimethyl sebacate comprising subjecting to an electrolytic condensation a solution of monomethyl adipate and an alkali salt thereof in methanol, the improvement which comprises maintaining the concentration of dimethyl sebacate in the solution at a level of at least 5% by weight; passing the solution through the electrolytic cell at a liquid velocity of at least 2.5 mlsee; withdrawing the resulting electrolytic solution containing dimethyl sebacate and combining it with an organic solvent capable of dissolving dimethyl sebacate and insoluble in water and with water to separate the withdrawn electrolytic solution into an aqueous layer substantially free of dimethyl sebacate and an organic solvent layer containing the dimethyl sebacate.

7. The improved process according to claim 6 wherein the organic solvent is a member selected from the group consisting of n-hexane, n-heptane, n-octane, isooctane and cyclohexane.

8. The improved process according to claim 6 wherein the water is added in an amount of 0.2 to 2 parts by weight per part by weight of the withdrawn electrolytic solution.

9. The improved process according to claim 6 wherein the organic solvent is added in an amount of 0.2 to 2 parts by weight per part by weight of the withdrawn electrolytic solution.

10. The improved process according to claim 6 wherein the withdrawn electrolytic solution is first combined with the organic solvent to effect separation of an organic solvent layer to which the water is added.

11. In a process for preparing sebaeic acid comprising subjecting to an electrolytic condensation a solution of monomethyl adipate and an alkali salt thereof in methanol to produce dimethyl sebacate and thereafter hydrolyzing the dimethyl sebacate to sebacic acid, the improvement which comprises maintaining the concentration of dimethyl sebacate in the solution at a level of at least 5% by weight; passing the solution through the electrolytic cell at a liquid velocity of at least 2.5 m/sec.; withdrawing the resulting electrolytic solution containing dimethyl sebacate and combining it with an organic solvent capable of dissolving dimethyl sebacate and insoluble in water and with water to separate the withdrawn electrolytic solution into an aqueous layer and an organic solvent layer whereby the dimethyl sebacate is extracted in the organic solvent layer; and hydrolyzing the separated dimethyl sebacate using an aqueous solution of a mineral acid in which dimethyl sebacate is present in an amount of at most the amount corresponding to the saturation solubility of sebacic acid in the aqueous solution of mineral acid at the hydrolysis temperature while removing methanol formed by the hydrolysis reaction from the reaction system.

12. The improved process according to claim ll wherein the mineral acid is nitric acid.

13. The improved process according to claim ll wherein the hydrolysis reaction is carried out at a temperature of about C. to C.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,896,011 DATED July 22, 1975 INVENTOR( Toshiro Isoya et 31 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 9, correct spelling of "rate".

Column 10, line 67, after "S" cancel and substitute therefor Column 16, claim 5,

line 3, cancel "4" and substitute therefor 20 Signed and Scaled this A ttes t:

RUTH C. MASON Arresting Officer C. MARSHALL DANN (nmmisximwr njParents and Trademarks 

1. IN A PROCESS FOR PREPARING DIMETHYL SEBACATE COMPRISING SUBJECTING TO AN ELECTROLYTIC CONDENSATION IN AN ELECTROLYTIC CELL A SOLUTION OF MONOMETHYL ADIPATE AND AN ALKALI SALT THEREOF IN METHANOL, THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE CONCENTRATION OF DIMETHYL SEBACATE IN THE SOLUTION AT A LEVEL OF AT LEAST 5% BY WEIGHT, AND PASSING THE SOLUTION THROUGH THE CELL AT A LIQUID VELOCITY OF AT LEAST 2.5 M/SEC.
 2. The improved process according to claim 1 whereiN the concentration of dimethyl sebacate is maintained at 5 to 40% by weight.
 3. The improved process according to claim 1 wherein the degree of neutralization of said solution is at most 60 mole %.
 4. The improved process according to claim 1 wherein the liquid velocity of the solution is 2.5 to 4.0 m/sec.
 5. The improved process according to claim 1 wherein the concentration of dimethyl sebacate in the solution is 4 to 40% by weight.
 6. In a process for preparing dimethyl sebacate comprising subjecting to an electrolytic condensation a solution of monomethyl adipate and an alkali salt thereof in methanol, the improvement which comprises maintaining the concentration of dimethyl sebacate in the solution at a level of at least 5% by weight; passing the solution through the electrolytic cell at a liquid velocity of at least 2.5 m/sec.; withdrawing the resulting electrolytic solution containing dimethyl sebacate and combining it with an organic solvent capable of dissolving dimethyl sebacate and insoluble in water and with water to separate the withdrawn electrolytic solution into an aqueous layer substantially free of dimethyl sebacate and an organic solvent layer containing the dimethyl sebacate.
 7. The improved process according to claim 6 wherein the organic solvent is a member selected from the group consisting of n-hexane, n-heptane, n-octane, isooctane and cyclohexane.
 8. The improved process according to claim 6 wherein the water is added in an amount of 0.2 to 2 parts by weight per part by weight of the withdrawn electrolytic solution.
 9. The improved process according to claim 6 wherein the organic solvent is added in an amount of 0.2 to 2 parts by weight per part by weight of the withdrawn electrolytic solution.
 10. The improved process according to claim 6 wherein the withdrawn electrolytic solution is first combined with the organic solvent to effect separation of an organic solvent layer to which the water is added.
 11. In a process for preparing sebacic acid comprising subjecting to an electrolytic condensation a solution of monomethyl adipate and an alkali salt thereof in methanol to produce dimethyl sebacate and thereafter hydrolyzing the dimethyl sebacate to sebacic acid, the improvement which comprises maintaining the concentration of dimethyl sebacate in the solution at a level of at least 5% by weight; passing the solution through the electrolytic cell at a liquid velocity of at least 2.5 m/sec.; withdrawing the resulting electrolytic solution containing dimethyl sebacate and combining it with an organic solvent capable of dissolving dimethyl sebacate and insoluble in water and with water to separate the withdrawn electrolytic solution into an aqueous layer and an organic solvent layer whereby the dimethyl sebacate is extracted in the organic solvent layer; and hydrolyzing the separated dimethyl sebacate using an aqueous solution of a mineral acid in which dimethyl sebacate is present in an amount of at most the amount corresponding to the saturation solubility of sebacic acid in the aqueous solution of mineral acid at the hydrolysis temperature while removing methanol formed by the hydrolysis reaction from the reaction system.
 12. The improved process according to claim 11 wherein the mineral acid is nitric acid.
 13. The improved process according to claim 11 wherein the hydrolysis reaction is carried out at a temperature of about 90*C. to 120*C. 